The basic design of most fundus imagers (both flood illuminating and scanning types) relies on a single concept that illuminating light is scattered by the retina (fundus) of the eye of a subject, and the return light is collected and detected. Thus, both illuminating and detected light must travel through the optical components of the eye, although not necessarily through common paths.
Diffuse and specular reflections from various optical components of the fundus imagers as well as the optical components of the eye such as the corneal and crystalline lens surface can contribute to unwanted light. Such scattered light can reduce the contrast required to detect low-level features in the fundus of an eye. For example, only about 1% of the incident light is scattered by the retinal tissue to contribute to the image, and of that, only about 1% is at angles that emerge from the pupil and are accessible for collection. Such a weak signal is easily overcome by the backscattered signal from the optical train of the instrument (˜10−2) and even more so from the unwanted reflections from the cornea (˜2×10−2). Even when the components of the optical system (such as the scan lens and the objective lens) have proper antireflection coatings, the various contributions to the unwanted reflections, also known as artifacts, easily overwhelm that of the desired signal. Thus the elimination of these unwanted artifacts has consumed considerable design efforts in ophthalmic instrumentation. Some practical known methods are pupil-splitting, anti-reflection points, and confocal apertures.
Broad-line fundus imaging (see e.g. WO2014/140256 hereby incorporated by reference) is one type of scan-based fundus imaging technology impacted by reflection artifacts. Pupil splitting and dead zones between illumination and detection zones (where ‘dead zones’ indicate un-used zones for either illumination or imaging) have been applied to these systems to reduce the impact of artifacts to some success. FIG. 1 diagrams two possible design approaches to a broad-line fundus imager. Each design has separate paths for illumination and collection. Light passes through an illumination aperture and then travels towards the retina. Light returning from the retina travels along the collection path towards the detector passing through a collection aperture. The scan lens (101=SL) and the objective lens (102=OL) are indicated for reference. In FIG. 1a, the collection aperture 103 is fixed while in FIG. 1b, the collection aperture 104 is a hole in the galvo scanner so its size varies depending on the scanner position. The collection aperture and sensitive region on the detector together define the region between the collection rays where the light must originate to be detected. Note that in these diagrams, the separation between illumination and collection beams has been exaggerated. Typically, the pupil size is on the order of a few millimeters, thus the separation of illumination and collection rays will be about a millimeter apart on the lenses.